﻿// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

namespace Microsoft.DotNet.Cli.Commands.New.MSBuildEvaluation;

/// <remarks>
/// As implemented in: https://docs.microsoft.com/en-us/dotnet/api/microsoft.visualstudio.shell.interop.vsprojectcapabilityexpressionmatcher?
/// </remarks>
internal class CapabilityExpressionEvaluator
{
    /// <summary>
    /// The set of terms that are present.
    /// </summary>
    private readonly IReadOnlyList<string> _presentTerms;

    /// <summary>
    /// The tokenizer that reads the expression.
    /// </summary>
    private readonly Tokenizer _tokenizer;

    /// <summary>
    /// The set of disallowed characters in terms.
    /// </summary>
    /// <remarks>
    /// We restrict many symbols, especially mathematical symbols, because we may eventually want to
    /// support arithmetic expressions.
    /// </remarks>
    internal static readonly char[] DisallowedCharacters = "\"'`:;,+-*/\\!~|&%$@^()={}[]<>? \t\b\n\r".ToCharArray();

    /// <summary>
    /// Initializes a new instance of the <see cref="CapabilityExpressionEvaluator"/> class.
    /// </summary>
    /// <param name="expression">The expression.</param>
    /// <param name="presentTerms">The present terms.</param>
    private CapabilityExpressionEvaluator(string expression, IReadOnlyList<string> presentTerms)
    {
        _tokenizer = new Tokenizer(expression);
        _presentTerms = presentTerms ?? throw new ArgumentNullException(nameof(presentTerms));
    }

    /// <summary>
    /// Evaluates the given expression against the given set of true terms. Missing terms are assumed to be false.
    /// </summary>
    /// <param name="expression">
    /// The expression, such as "(VisualC | CSharp) + (MSTest | NUnit)".
    /// The '|' is the OR operator.
    /// The '&' and '+' characters are both AND operators.
    /// The '!' character is the NOT operator.
    /// Parentheses force evaluation precedence order.
    /// A null or empty expression is evaluated as true.
    /// </param>
    /// <param name="presentTerms">The terms that are currently defined.</param>
    /// <returns>The result of evaluating the Boolean expression.</returns>
    public static bool Evaluate(string expression, IReadOnlyList<string> presentTerms)
    {
        if (string.IsNullOrWhiteSpace(expression))
        {
            // An empty expression evaluates to true.
            return true;
        }

        var eval = new CapabilityExpressionEvaluator(expression, presentTerms);
        return eval.Top();
    }

    /// <summary>
    /// Checks whether a given character is an allowed member of a term.
    /// </summary>
    /// <param name="ch">The character to test.</param>
    /// <returns>true if the character would be an allowed member of a term; false otherwise.</returns>
    private static bool IsSymbolCharacter(char ch)
    {
        return !DisallowedCharacters.Contains(ch);
    }

    /// <summary>
    /// Processes | operators.
    /// </summary>
    /// <returns>The result of evaluating the current sub-expression.</returns>
    private bool OrTerm()
    {
        bool lhs = AndTerm();
        while (_tokenizer.Peek() == "|")
        {
            _tokenizer.Next();
            bool rhs = AndTerm();
            lhs = lhs || rhs;
        }

        return lhs;
    }

    /// <summary>
    /// Processes &amp; operators.
    /// </summary>
    /// <returns>The result of evaluating the current sub-expression.</returns>
    private bool AndTerm()
    {
        bool lhs = Term();
        while (_tokenizer.Peek() == "&")
        {
            _tokenizer.Next();
            bool rhs = Term();
            lhs = lhs && rhs;
        }

        return lhs;
    }

    /// <summary>
    /// Processes terms.
    /// </summary>
    /// <returns>The result of evaluating the current sub-expression.</returns>
    private bool Term()
    {
        int notCount = 0;
        while (_tokenizer.Peek() == "!")
        {
            _tokenizer.Next();
            notCount++;
        }

        if (_tokenizer.Peek() == "(")
        {
            _tokenizer.Next();
            bool r = OrTerm();
            if (_tokenizer.Peek() != ")")
            {
                throw _tokenizer.CreateInvalidExpressionException();
            }
            _tokenizer.Next();
            return notCount % 2 == 0 ? r : !r;
        }
        else if (_tokenizer.Peek() != null && IsSymbolCharacter(_tokenizer.Peek()![0]))
        {
            string? ident = _tokenizer.Next();
            bool isPresent = _presentTerms.Contains(ident, StringComparer.OrdinalIgnoreCase);
            return notCount % 2 == 0 ? isPresent : !isPresent;
        }
        else
        {
            throw _tokenizer.CreateInvalidExpressionException();
        }
    }

    /// <summary>
    /// Processes the entire expression.
    /// </summary>
    /// <returns>The result of evaluating the expression.</returns>
    private bool Top()
    {
        bool r = OrTerm();
        if (_tokenizer.Peek() != null)
        {
            throw Tokenizer.CreateInvalidExpressionException(_tokenizer.Input.Length);
        }

        return r;
    }

    /// <summary>
    /// The expression tokenizer.
    /// </summary>
    /// <devremarks>
    /// This is a struct rather than a class to avoid allocating memory unnecessarily.
    /// </devremarks>
    private class Tokenizer
    {
        /// <summary>
        /// The most recently previewed token.
        /// </summary>
        private string? _peeked;

        /// <summary>
        /// Initializes a new instance of the <see cref="Tokenizer"/> class.
        /// </summary>
        /// <param name="input">The expression to parse.</param>
        internal Tokenizer(string input)
        {
            if (string.IsNullOrEmpty(input))
            {
                throw new ArgumentException($"'{nameof(input)}' cannot be null or empty.", nameof(input));
            }
            Input = input;
        }

        /// <summary>
        /// Gets the entire expression being tokenized.
        /// </summary>
        internal string Input { get; }

        /// <summary>
        /// Gets the position of the next token.
        /// </summary>
        internal int Position { get; private set; }

        /// <summary>
        /// Gets the next token in the expression.
        /// </summary>
        internal string? Next()
        {
            // If the last call to Next() was within a Peek() method call,
            // we need to return the same value again this time so that
            // the Peek() doesn't impact the token stream.
            if (_peeked != null)
            {
                string token = _peeked;
                _peeked = null;
                return token;
            }

            // Skip whitespace.
            while (Position < Input.Length && char.IsWhiteSpace(Input[Position]))
            {
                Position++;
            }

            if (Position == Input.Length)
            {
                return null;
            }

            switch (Input[Position])
            {
                case char sym when IsSymbolCharacter(sym):
                    int begin = Position;
                    while (Position < Input.Length && IsSymbolCharacter(Input[Position]))
                    {
                        Position++;
                    }
                    int end = Position;
                    return Input.Substring(begin, end - begin);
                // we prefer & but also accept + so that XML manifest files don't have to write the &amp; escape sequence.
                case '&':
                case '+':
                    Position++;
                    return "&";  // always return '&' to simplify the parser logic by consolidating on only one of the two possible operators.
                case '|':
                    Position++;
                    return "|";
                case '(':
                    Position++;
                    return "(";
                case ')':
                    Position++;
                    return ")";
                case '!':
                    Position++;
                    return "!";
                default: throw CreateInvalidExpressionException(Position);
            }
        }

        /// <summary>
        /// Peeks at the next token in the stream without skipping it on
        /// the next invocation of <see cref="Next"/>.
        /// </summary>
        internal string? Peek()
        {
            return _peeked = Next();
        }

        /// <summary>
        /// Create an exception indicating that the expression is invalid and reporting the current position.
        /// </summary>
        /// <param name="position">The position in the expression where the error was detected.</param>
        /// <returns>An exception for diagnosing the invalid expression.</returns>
        internal Exception CreateInvalidExpressionException()
        {
            return CreateInvalidExpressionException(Position);
        }

        /// <summary>
        /// Create an exception indicating that the expression is invalid and reporting the given position.
        /// </summary>
        /// <param name="position">The position in the expression where the error was detected.</param>
        /// <returns>An exception for diagnosing the invalid expression.</returns>
        internal static Exception CreateInvalidExpressionException(int position)
        {
            return new ArgumentException(
                string.Format(CliCommandStrings.CapabilityExpressionEvaluator_Exception_InvalidExpression, position),
                "expression");
        }
    }
}
